Reverse link transmissions may require transmission of a pilot signal from the mobile stations to allow for the receiver in the base station to perform coherent multi-path combining and demodulation. Generally, to find an optimum power level for the total transmission power level of the pilot channel and data channels, the power level for the pilot channel is minimized while trying to achieve a decoding error rate performance. For example, in a system commonly known as cdma2000 1x, for the 9600 bit/s format at 1% frame error rate (FER), the optimum pilot channel power level is found experimentally to be about 3.75 dB lower than the data channels power level. If the power level of the pilot channel is increased much above such a defined level, the overall decoding performance does not significantly improve, even though the total transmission power for pilot channel and the data channels is higher. On the other hand, if the pilot channel power level is decreased much below such a defined level, the data channels power level need to be increased to achieve the same decoding error rate performance. In such a case, the total power level for the pilot channel and the data channels is also higher. Therefore, there is an optimum pilot channel power level with respect to a data channel power level for a communication data rate at a decoding error rate performance level. The following graph may illustrate the pilot channel optimum power level with respect to the total power level used for transmission of the pilot channel and the data channels. The optimum pilot power level may be different for different data rates. Higher data rate transmissions have an optimum pilot level that may be much higher than the pilot level required for low data rates. The difference in optimal pilot levels for low and high data rates may be about 13 dB.
The pilot channel power level is also measured by the receiver in a power control process for controlling the transmission power level. Typically, the receiver measures the signal to noise ratio (SNR) of the pilot channel. The measured SNR is compared to a threshold. If the measured SNR is higher than the threshold, the receiver through its accompanying transmitter instructs the transmitting source to lower the pilot channel power. The data channel power level is also lowered to maintain a pilot channel to data channel power level ratio. If the measured SNR is lower than the threshold, the receiver through its accompanying transmitter instructs the transmitting source to increase the pilot channel power. The data channel power level is also increased to maintain a pilot channel to data channel power level ratio. As such, the receiving end through the power control process attempts to maintain a pilot SNR at the receiver for proper decoding process at a minimal error rate.
The communication system also has a data rate control process that attempts to maximize the transmission data rate for optimum data throughput. Based on measured channel characteristics, the data rate may be increased or lowered. In another aspect, the data rate may change based on the demand, considering that the channel characteristics allow for proper communications at the requested data rate.
In such a communication system, the pilot channel power control and the data rate control may operate independently. As such, when the data rate is changed, the power level of the pilot channel may also change, without the power control process involvement, to maintain the optimum pilot channel power level. Since the power control process has no knowledge of the data rate change and the corresponding pilot channel power change, the power control process may take the change in the pilot channel power as a change in the channel propagation. Such a detection normally initiates a process for changing the pilot channel power through the power control process. Therefore, if the change in the pilot channel power level to satisfy a different data rate transmission is without notifying the receiving end in advance, the power control process may erroneously instruct for the pilot channel to rectify its transmit power.
Therefore, there is a need for power control process and data rate control process to operate simultaneously in a communication system without any adverse effect.